Data communications by wireless LAN such as IEEE802.11 are now widely used, for instance, in personal computers (PCs), PC peripherals such as printers, hard disk drives and broadband routers, electronic apparatuses such as FAXs, refrigerators, standard televisions (SDTVs), high-definition televisions (HDTVs), digital cameras, digital video cameras and mobile phones, signal-transmitting means in automobiles and aircrafts, etc.
As the wireless LAN standard, IEEE802.11a uses an orthogonal frequency division multiplexing (OFDM) modulation system in a frequency band of 5 GHz, supporting high-speed data communications of maximum 54 Mbps. IEEE802.11b uses a direct sequence spread spectrum (DSSS) system in an industrial, scientific and medical (ISM) band of 2.4 GHz usable without a wireless license, supporting high-speed communications of 5.5 Mbps and 11 Mbps. IEEE802.11g uses the OFDM modulation system in a 2.4-GHz band like IEEE802.11b, supporting high-speed data communications of maximum 54 Mbps.
As a high-frequency circuit for use in a multi-band communications apparatus using such wireless LAN, WO2006/003959A discloses a high-frequency circuit capable of performing diversity reception in a multiband communications apparatus for two communications systems (IEEE 802.11a and IEEE 802.11b) of a 2.4-GHz band and a 5-GHz band in wireless LAN. This high-frequency circuit comprises, as shown in FIG. 35,                a diplexer circuit 13 between a high-frequency switch circuit 10 and a transmission circuit;        a power amplifier circuit 2 and a bandpass filter circuit 4 between the diplexer circuit 13 and a transmission terminal 11bg-T;        a lowpass filter circuit 19, a power amplifier circuit 3 and a bandpass filter circuit 5 between the diplexer circuit 13 and a transmission terminal 11a-T;        a detection circuit 8 between the high-frequency switch circuit 10 and the diplexer circuit 13;        a diplexer circuit 14 between the high-frequency switch circuit 10 and a receiving circuit;        a bandpass filter circuit 6 between the diplexer circuit 14 and a receiving terminal 11bg-R;        a lowpass filter circuit 26 and a low-noise amplifier circuit 27 between the diplexer circuit 14 and a receiving terminal 11a-R;        a notch circuit 28 between an antenna terminal Ant1 and the high-frequency switch circuit 10; and        a notch circuit 29 between an antenna terminal Ant2 and the high-frequency switch circuit 10.        
WO 2006/003959 also discloses an example comprising low-noise amplifier circuits in paths connected to a receiving terminal 11bg-R for a 2.4-GHz band and a receiving terminal 11a-R for a 5-GHz band. In the high-frequency circuit of WO 2006/003959, the diplexer circuit is disposed on the input side of the low-noise amplifier, and the bandpass filter or the lowpass filter is connected between the diplexer circuit and the low-noise amplifier.
As a high-frequency circuit commonly usable for wireless LAN and Bluetooth, JP2002-208874A discloses, as shown in FIG. 36, a high-frequency circuit comprising a bandpass filter 2 between an antenna 1 and an antenna switch 3, a power amplifier circuit 5 disposed on the transmission side of the antenna switch 3 for being commonly used for wireless LAN and Bluetooth, a diplexer (combination of a lowpass matching circuit 13 and a highpass matching circuit 14) connected to the power amplifier circuit 5 for branching transmission for wireless LAN and transmission for Bluetooth, a low-noise amplifier 7 disposed on the receiving side of the antenna switch 3 for being commonly used for wireless LAN and Bluetooth, and a diplexer (combination of a lowpass matching circuit 15 and a highpass matching circuit 16) connected to the low-noise amplifier 7 for branching reception for wireless LAN and reception for Bluetooth.
The receiving sensitivity is largely influenced by the noise index of a low-noise amplifier and the insertion loss of a bandpass filter and a diplexer circuit. With respect to the reduction of the noise index of the low-noise amplifier, minimizing loss in its input stage is most effective. However, the structure of the high-frequency circuit of WO 2006/003959 fails to obtain sufficient improvement in the receiving sensitivity. Also, because a switch circuit, etc. are vulnerable to electrostatic surge, the switch circuit, etc. are likely broken in the circuit structure of WO 2006/003959, when electrostatic discharge (ESD) is applied to the antenna.
In the high-frequency circuit of WO 2006/003959, bias voltage of about several mA should be supplied to the power amplifier circuit and the low-noise amplifier circuit, but a logic control power supply integrated with an RFIC or a baseband IC cannot directly be driven because its driving current is 2 mA or less.
In the high-frequency circuit of JP 2003-273687 A, the highpass filter circuit is connected to the antenna terminal to prevent the breakage of the switch circuit, etc. by electrostatic discharge. However, when the function of transmitting and receiving a wireless LAN is added to a mobile gear such as a cell phone, part of a transmitting signal for the mobile gear is likely to enter the wireless LAN system, saturating particularly a low-noise amplifier in a receiving path, and thus deteriorating the receiving sensitivity. The circuit structure of JP 2003-273687 A coping with electrostatic discharge cannot fully solve such problem. In the high-frequency circuit of JP 2002-208874 A, one bandpass filter disposed between an antenna and an antenna switch conducts the reduction of harmonics generated on the transmission side and the attenuation of noise on the receiving side, but it cannot be used commonly for two frequencies of a 2.4-GHz band and a 5-GHz band.
Further, wireless LAN communications apparatuses for IEEE 802.11n according to a multi-input-multi-output (MIMO) technology for increasing the speed and quality of communications by using pluralities of antennas have been finding wider applications. However, the high-frequency circuits of WO 2006/003959 and JP 2003-273687 A cannot fully handle IEEE 802.11n.